Cacher is the code snippet organizer for pro developers

The keyboard combination to display the current line number whilst you are using nano is CTRL+C.

Alternatively, to display the line & column number position you could just use the -c parameter when launching nano i.e:

nano -c [filename]
To make this permanent, nano uses a configuration in your home folder ~/.nanorc

Thus to display line numbers always when using nano...

nano ~/.nanorc
(don't worry if its empty - this file doesn't exist by default)

type set const